This invention relates generally to cyclic pulsed detonation combustors (PDCs) and more particularly, to a compact, low pressure drop transition of detonations with small initiation devices.
In a generalized pulse detonation combustor, fuel and oxidizer (e.g., oxygen-containing gas such as air) are admitted to an elongated combustion chamber at an upstream inlet end of the pulse detonation combustor. An igniter (spark or plasma ignitor) is used to initiate a combustion process within the pulse detonation combustor. Following a successful transition to detonation, a detonation wave propagates toward an outlet of the pulse detonation combustor at supersonic speed causing a substantial combustion of the fuel and oxidizer mixture before the mixture can be substantially driven from the outlet. A result of the combustion is to rapidly elevate pressure within the pulse detonation combustor before a substantial amount of gas can escape through the outlet. An effect of this inertial confinement is to produce near constant volume combustion. The pulse detonation combustor can be used to produce pure thrust or can be integrated in a gas-turbine engine. The former is generally termed a pure thrust-producing device and the latter is generally a hybrid engine device. A pure thrust-producing device is often used in a subsonic or supersonic propulsion vehicle system, such as, rockets, missiles, and an afterburner of a turbojet engine. Industrial gas turbines are often used to provide output power to drive an electrical generator or motor. Other types of gas turbines may be used as aircraft engines, on-site and supplemental power generators, and for other applications.
A deflagration-to-detonation transition (DDT) process begins when a mixture of fuel and air in the chamber is ignited via a spark, laser or other source. A subsonic flame kernel generated from the ignition accelerates as the subsonic flame travels along the length of the chamber due to chemical processes and flow mechanics. As the subsonic flame reaches critical supersonic speeds, “hot spots” are created that create localized explosions, eventually transitioning the subsonic flame to a super-sonic detonation wave. The DDT process can take up to several meters of the length of the chamber, and efforts have been made to reduce the distance used for DDT by using internal obstacles, such as orifice plates or spirals, in the flow of a mixture of fuel and oxidizer within the chamber. However, the obstacles for cyclic detonation devices have a relatively high pressure drop and are cooled. Moreover, the detonation initiation, in the chamber with obstacles, occurs within a run-up length which ranges from and including 15 to 20 times a diameter of the chamber, and thus the run-up length increases with increasing chamber diameter. For practical propulsion systems, the run-up length due to this constraint can be prohibitively long.